Back in the day it wasn't a bug, but rather more of a cheat to make your system feel faster than it really was: if the system actually gave the UI a fair share of CPU cycles, the system would feel incredibly sluggish. Conversely, if it gave the UI the lions share of CPU, nothing would ever get done in the background. So it compromised and gave the UI preferential treatment when the user was active (i.e. when there was recent mouse/keyboard activity), and a more fair allocation when they weren't so other things could get done.